1. Field of the Invention
The present invention relates to a multistage amplifier apparatus with a distortion compensation function.
2. Description of the Background Art
For a transmitting apparatus or the like for performing communication using a radio frequency signal, an amplifier apparatus for amplifying a transmission signal is used. Generally, when a signal is amplified by an amplifier which constitutes the amplifier apparatus, the amplified signal is distorted, and a linear relationship between an input signal and an amplified output signal cannot be ensured. Therefore, in order to solve this problem, there is a method which provides a distortion compensation function to the amplifier. Further, there is a method in which, for the purpose of expanding a dynamic range of the output signal, a plurality of the amplifiers serially connected to one another is provided to the transmitting apparatus or the like, whereby a power gain is increased gradually. These methods may be also used in the case of amplifying a signal using a polar modulation.
FIG. 9 is a diagram showing an exemplary polar modulation amplifier apparatus 70 for amplifying a signal using a polar modulation. As with the polar modulation shown in FIG. 9, an inputted transmission signal is once separated into an amplitude component and a phase component, and then processed, whereby it is possible to realize a power saving amplifier apparatus having a small-circuit scale.
FIG. 10 is a conventional amplifier apparatus including a distortion compensation function which is disclosed in the U.S. Pat. No. 6,191,653 (Patent Document 1). The conventional amplifier apparatus disclosed in Patent Document 1 includes one amplifier, and performs distortion compensation using a feedback control.
FIG. 11 shows another conventional amplifier apparatus which is disclosed in the U.S. Pat. No. 6,256,482 (Patent Document 2) and has a plurality of amplifiers serially connected to one another. The conventional amplifier apparatus disclosed in Patent Document 2 uses the polar modulation, and the power gain of a transmission signal is gradually increased by two amplifiers.
Based on the above-described conventional techniques, a multistage amplifier apparatus having a distortion compensation function shown in FIGS. 12 and 13 may be devised easily. FIG. 12 is a diagram showing an exemplary configuration of a conventional multistage amplifier apparatus 100 using the polar modulation. As shown in FIG. 12, the conventional multistage amplifier apparatus 100 includes conversion circuits 54 and 55, a LUT (look up table) 53, and an amplifier section 56 constituted of an amplifier 51 and an amplifier 52 which are serially connected to each other. To a power supply terminal 61 of the amplifier 51, an output of the conversion circuit 54 is connected. To a power supply terminal 62 of the amplifier 52, an output of the conversion circuit 55 is connected. To an input of the conversion circuit 54 and an input of the conversion circuit 55, an output of the LUT 53 is connected.
Hereinafter, an operation of the conventional multistage amplifier apparatus 100 will be described. As shown in FIG. 12, a voltage of an amplitude signal AM11 to be inputted to the LUT 53 is represented by Vramp11, a voltage of an amplitude signal AM12 outputted by the LUT 53 is represented by Vramp12, a voltage of an amplitude signal AM13 outputted by the conversion circuit 54 is represented by Vramp13, a voltage of an amplitude signal AM14 outputted by the conversion circuit 55 is represented by Vramp14, and a voltage of an output signal Pout outputted by the amplifier section 56 is represented by Vout.
The LUT 53 outputs the voltage Vramp12 which allows a linear relationship between the voltage Vramp11 and the output voltage Vout in accordance with a table described hereinbelow.
FIG. 14 is an exemplary diagram showing a relation among the output voltage Vout, the voltage Vramp11, and the voltage Vramp12. As shown in FIG. 14, there is a value of the voltage Vramp12 which can ensure the linear relationship between the voltage Vramp11 and the output voltage Vout. Accordingly, the LUT 53 previously retains a table, which is prepared based on FIG. 14 and represents the relation between the voltage Vramp11 and the voltage Vramp12, and outputs the voltage Vramp12 corresponding to the voltage Vramp11 in accordance with the table, whereby it is possible to compensate for distortion of the amplifier section 56.
Next, the conversion circuit 54 converts the voltage Vramp12 to be inputted thereto, and provides the power supply terminal 61 of the amplifier 51 with the voltage Vramp13 for an optimal driving condition which is determined based on a relation between the amplifier 51 and the amplifier 52. The optimal driving condition is a condition which causes power efficiency, impedance, optimal saturation, and the like of the conventional multistage amplifier apparatus 100 to be comprehensively optimal. In a similar manner, the conversion circuit 55 converts the voltage Vramp12 to be inputted thereto, and provides the power supply terminal 62 of the amplifier 52 with the voltage Vramp14 for the optimal driving condition which is determined based on the relation between the amplifier 51 and the amplifier 52.
Next, the amplifier 51 performs amplitude modulation on an input signal Pin in accordance with the voltage Vramp13 provided to the power supply terminal 61, and outputs the amplitude-modulated signal to the amplifier 52. The amplifier 52 performs the amplitude modulation on the amplitude-modulated signal inputted by the amplifier 51 in accordance with the voltage Vramp14 provided to the power supply terminal 62, and outputs this further amplitude-modulated signal as the output signal Pout of the conventional multistage amplifier apparatus 100.
In this manner, in the conventional multistage amplifier apparatus 100, the input signal Pin is gradually amplified by a plurality of the amplifiers 51 and 52. Further, the LUT 53 performs the distortion compensation on the input signal Pin, whereby it is possible to realize less distorted power amplification.
FIG. 13 is a diagram showing an exemplary configuration of a conventional multistage amplifier apparatus 200 using the polar modulation. The conventional multistage amplifier apparatus 200 has a configuration in which the LUT 53, the conversion circuit 54, and the conversion circuit 55 of the conventional multistage amplifier apparatus 100 are replaced with a conversion circuit 57, a LUT 58, and a LUT 59, respectively.
Hereinafter, an operation of the conventional multistage amplifier apparatus 200 will be described. As shown in FIG. 13, a voltage of an amplitude signal AM11 inputted to the conversion circuit 57 is represented by Vramp11, a voltage of an amplitude signal AM15 outputted by the conversion circuit 57 to the LUT 58 is represented by Vramp15, a voltage of an amplitude signal AM16 outputted by the conversion circuit 57 to the LUT 59 is represented by Vramp16, a voltage of an amplitude signal AM17 outputted by the LUT 58 is represented by Vramp17, a voltage of an amplitude signal AM18 outputted by the LUT 59 is represented by Vramp18, and a voltage of an output signal Pout by the amplifier section 56 is represented by Vout.
The conversion circuit 57 converts the voltage Vramp11 to be inputted thereto, and outputs the voltage Vramp15 and the voltage Vramp16 for an optimal driving condition, which are determined based on the relation between the amplifier 51 and the amplifier 52, to the LUT 58 and the LUT 59, respectively. The optimal driving condition is the same as that described in the description of the conventional multistage amplifier apparatus 100. Next, the LUT 58 provides the voltage Vramp17, which allows a linear relationship between the voltage Vramp15 and a voltage of an output signal from the amplifier 51, to a power supply terminal 61 of the amplifier 51. In a similar manner, the LUT 59 provides the voltage Vramp18, which allows a linear relationship between the voltage Vramp16 to be inputted thereto and a voltage of an output signal from the amplifier 52, to a power supply terminal 62 of the amplifier 52.
In the same manner as the LUT 53 of the conventional multistage amplifier apparatus 100, the LUT 58 previously retains a table which represents a relation between the voltage Vramp15 and the voltage Vramp17 and which allows the linear relationship between the voltage Vramp15 and the voltage of the output signal from the amplifier 51. In a similar manner, the LUT 59 previously retains a table which represents a relation between the voltage Vramp16 and the voltage Vramp18 and which allows the linear relationship between the voltage Vramp16 and the voltage of the output signal from the amplifier 52.
Next, the amplifier 51 performs amplitude modulation on the input signal Pin in accordance with the voltage Vramp17 provided to the power supply terminal 61, and outputs the amplitude-modulated signal to the amplifier 52. The amplifier 52 then performs the amplitude modulation on the amplitude-modulated signal inputted by the amplifier 51 in accordance with the voltage Vramp18 provided to the power supply terminal 62, and outputs the further amplitude-modulated signal as an output signal Pout of the conventional multistage amplifier apparatus 200.
In this manner, in the conventional multistage amplifier apparatus 200, the input signal Pin is gradually amplified by a plurality of the amplifiers 51 and 52. Further, the LUT 58 and the LUT 59 perform distortion compensation on the input signal Pin, whereby it is possible to realize less distorted power amplification.
However, in the above-described conventional multistage amplifier apparatus 100, in order to provide an amplitude signal voltage for the optimal driving condition to the power supply terminal 61 of the amplifier 51 and the power supply terminal 62 of the amplifier 52, the same number of conversion circuits 54 and 55 as the amplifiers 51 and 52 are required. Further, the LUT 53 is required which improves a linearity characteristic between the input and the output of the conventional multistage amplifier apparatus 100.
Accordingly, the conventional multistage amplifier apparatus 100 has a problem in that a circuit scale, manufacturing costs, and power consumption thereof increase.
Further, in the above-described conventional multistage amplifier apparatus 200, in order to provide the amplitude signal voltage for the optimal driving condition to the power supply terminal 61 of the amplifier 51 and the power supply terminal 62 of the amplifier 52, the conversion circuit 57 is required. Further, in order to improve a linearity characteristic between the input and output of the conventional multistage amplifier apparatus 200, the same number of LUTs 58 and 59 as the amplifiers 51 and 52 are required. Accordingly, the conventional multistage amplifier apparatus 200 also has a problem in that a circuit scale, manufacturing costs and power consumption thereof increase.